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For Aging Ocean Explorer, a New Life at New Depths

THE sea has millions of dark secrets. And now, a famous explorer is to illuminate more of them.

For more than a third of a century, Alvin, the world's first craft to roam the abyss with relative ease, has been ferrying American scientists into the planet's sunless depths and racking up a staggering record of discovery, including dark ecosystems that, with riots of tube worms and other bizarre creatures, rival rain forests in richness.

But the plucky, white, 25-foot-long craft has its shortcomings. Most notably, it can plunge down only so far without deep sea pressures suddenly smashing its crew capsule (and three occupants) into smithereens. For dives, the safety limit is 2.8 miles.

That distance is little more than the sea's average depth and far short of its deepest spot, seven miles down, much lower than Everest is high. The upshot is that a large part of the seabed, with its dark recesses and riddles and perhaps even a den or two of undiscovered monsters, is out of Alvin's reach. Off limits is an area larger than Asia.

But now, Alvin is getting a major overhaul that may extend its reach and allow it to match and possibly even surpass newer submersibles abroad, making it the world leader.

In a remarkable bonanza from the end of the cold war, the Woods Hole Oceanographic Institution, which runs Alvin, has received from the Navy a submersible known as Sea Cliff, which has been decommissioned in the wake of military cutbacks and changing priorities.

Woods Hole and its Federal patrons have insufficient money to run Sea Cliff, designed for military work, as a separate vehicle. But Woods Hole is happy to cannibalize its parts to improve Alvin, a process now getting under way. The trove is rich: Alvin cost about $50 million; Sea Cliff about $200 million.

The crown jewel of Sea Cliff is its crew capsule. The walls are made of a thick layer of super-strong titanium that can withstand pressures down to depths of 3.8 miles (or 6 kilometers). So Woods Hole is now studying whether the capsule can be transferred to Alvin, a move that would extend the reach of the tiny submersible by a full mile.

''We have a responsibility to make sure it's the best,'' Dr. Robert B. Gagosian, the director of Woods Hole, said in an interview.

Eager to push even deeper and capitalize on the politics of the moment, the institution is seeing whether further strengthening would allow Alvin to plunge to depths of 4.3 miles (or 7 kilometers), deeper than any of the rival submersibles now operated by Russia, France and Japan.

''We've been the leaders in exploration,'' Dr. Gagosian said. ''In order to continue that tradition, we need to have the deepest-diving submersible. It's that simple. We want to lead.''

Scientists say major questions of geology, biology and even archeology now lie beyond Alvin's current reach, not to mention the unknown.

''There's important science to be done'' in the sea's deeper regions, said Dr. Daniel J. Fornari, head scientist for the deep submergence program at Woods Hole. ''It's such a compelling part of the planet. We've got this sitting in our backyard and we know so little about it. It's ridiculous.''

Over the decades, Alvin users have been the first to glimpse the seabed's dark ecosystems, its hot chimneys, its cold seeps, its rocky chains of mountains longer than the Andes and its lost human worlds, such as the Titanic. The explorers have also brought to light thousands of new species of animal life, including slimy things seemingly fit for monster movies.

This record of ongoing excellence is causing some of the submersible's users to view any renovations warily.

''One thing is clear,'' said Richard F. Pittenger, head of marine operations at Woods Hole. ''We don't want to screw up the reliability in the process of trying to make the submersible better.''

When Alvin was first proposed by Woods Hole in the 1960's, it was considered a curiosity. Most military officials and scientists of the day cared little about exploring the deep. No regular Navy contractor wanted job. So the submersible was built by General Mills, the maker of breakfast cereals.

The situation changed after a Navy submarine, the Thresher, sank with 129 men in 1963. Its wreckage remained lost for months. In the catastrophe's wake, the Navy decided to throw itself into mastering the abyss.

No expense was too great. The Navy built two Alvin look-alikes, Sea Cliff and Turtle. Both were heavier and faster than their forerunner. Eventually, in 1984, Sea Cliff dove much deeper, with its stronger crew capsule.

However, foreign submersibles began to rival Sea Cliff's maximum depth of 3.8 miles and to make new discoveries. In 1990, the Japanese pulled ahead with a new submersible that plunged down 4 miles, a world record in the Pacific. Woods Hole scientists looked on sheepishly again in 1994 as the Japanese submersible, Shinkai 6500, set an Atlantic depth record at nearly 4 miles. Today, the 31-foot-long Japanese submersible is still unrivaled.

Such competitive pressure is one reason Woods Hole is eager to have Alvin go deeper. The more important reason is raw knowledge.

Scientists say the inky deep is not only inherently provocative but important to understand if people are to be good planetary stewards. It is the earth's largest habitat, containing, by some estimates, 97 percent of the space inhabited by living things.

It is also the planet's lifeblood. Its voluminous waters supply vital elements that sustain life on the surface and regulate planetary heat flow, keeping the land temperate.

Geologically, the deep is the planet's most dynamic feature, the place where huge slabs of crust are created and destroyed. About 90 percent of the Earth's volcanic activity happens undersea.

Dr. Gagosian, the Woods Hole director, said a deeper-diving Alvin could better study these cauldrons, which also support lush ecosystems and are suspected of being the place where life arose 3.8 billion years ago.

A deeper-diving Alvin, he said, could not only explore the sites directly but could wire the ocean with sensitive detectors meant to track distant seaquakes. A great opportunity for such wiring lies in old telephone cables that crisscross the sea's abyssal plains, Dr. Gagosian said.

This month, one such cable between California and Hawaii is to be lifted from a depth of 3.1 miles and fitted with geologic detectors, an oceanographic first. A deeper Alvin could maintain such devices, eliminating the need for lifting the cable, which is a risky job.

A deeper Alvin would also be able to visit more of humanity's lost worlds. The luxury liner, Titanic, resting at a depth of more than two miles, is now within Alvin's reach and was visited in 1986 by the tiny white submersible, but thousands of other wrecks lie deeper.

One is the I-52, a Japanese submarine sunk in World War II while carrying a shipment of two tons of gold, 146 bars of it packed in metal boxes. Discovered in 1995 more than three miles down on the Atlantic abyssal plain, the I-52 wreckage is now slated for further exploration as the salvage team searches for vehicles than can go that deep and are available for such work.

Bits of Sea Cliff are already being transferred to Alvin. The first to go was the military submersible's digital sonar.

Barrie B. Walden, manager of deep submergence at Woods Hole, said incorporating Sea Cliff's crew capsule into Alvin might cost $15 million, a sum as yet unapproved by Federal planners. He said the upgrade could be done during the sub's next major overhaul, which is scheduled for the end of 2000, allowing it to start the new millennium on an auspicious note.

With Alvin diving to depths of 3.8 miles (or 6 kilometers), the region outside its reach would shrink significantly, contracting from something larger than Asia to the size of Europe.

Dr. Gagosian, eager for even greater penetration of the sea's darkness, has asked Woods Hole experts to assess the feasibility of Alvin's plunging to depths of 4.3 miles (or 7 kilometers). But achieving that may take more than Sea Cliff's sphere, and might cost $100 million -- big money in a field that gets about $20 million a year in Federal money.

Some scientists feel such money is unlikely to materialize. But others are hopeful, saying that the potential returns are huge and the cost is extraordinarily low compared with other Federal science investments.

''It's way underfunded,'' Dr. Fornari said of the nation's efforts at deep exploration, still led by Alvin.